1. Field of the Invention
The present invention relates to a method for measuring temperature using a negative temperature coefficient sensor. More particularly, a method of this type is intended to be employed in the field of automobile electronics.
2. Description of the Related Art
It is already known to use sensors whose resistance varies as a function of the temperature of the medium in which they are immersed. By measuring this resistance variation, and after processing the measured signal by a common signal processing means, a digital value N is determined which represents the temperature prevailing around the sensor.
Conventionally, as regards temperature sensors with negative temperature coefficient (also referred to as NTC sensors), a voltage source VA is applied to a sensor and a resistor which is connected in series (referred to as a "pull up" resistor) is connected to the positive pole of the voltage source. An analog measured signal, taken from a midpoint lying between the resistor and the sensor, is sent to an analog/digital converter. This converter also receives a reference voltage, so as to generate a digital value N representing the temperature measured by the sensor.
However, devices of this type have a major drawback, namely: it is difficult for them to measure with high precision a temperature which varies over an extended range.
In order to overcome this drawback, it is known to switch the reference voltage of the converter between a plurality of different reference voltages. One measured temperature range corresponds to each reference voltage. This provides a plurality of measurement scales for accurately monitoring a temperature which varies over an extended range. For example, in the automotive field, the temperature to be measured may vary from -40.degree. to +140.degree. C.
The drawback of devices of this type with multiple reference voltages is that the converter must be provided with a plurality of accurately calibrated voltage sources. Such a multiplicity of calibrated voltage sources is expensive.
It is therefore desired to create a method for measuring temperature which uses only a single voltage source, while ensuring that the measurement is highly accurate over the entire extent of the temperature range.
It is already known, to this end, to vary the resistance of the resistor connected in series with the sensor (by switching a transistor connected in parallel with this resistor), in order to improve temperature monitoring on different temperature scales. However, this technique has the drawback of changing the current flowing in the measurement branch and, in particular, it introduces errors due to the voltage drops and the leakage currents of the transistors.